Comparability of High-Lift Noise Measurements in a Kevlar-Wall, a Hard-Wall and an Open-Jet Test Section.

Abstract

Aeroacoustic wind tunnel tests are a fundamental tool for fundamental aeroacoustic studies of single and multi-element airfoils. Hard-wall, open-jet, and hybrid wind tunnel test section types can nowadays be used for these type of studies. These test section configurations each have particular benefits and merits when it comes to their acoustic and aerodynamic behavior. In this paper, we investigated the comparability of leading-edge slat noise measurements in a hard-wall, an open-jet and a hybrid test section. This is the first time that measurements of the same 2D high-lift model are performed in the same wind tunnel with different test section types. These measurements were carried out with a 2D 30P30N high-lift airfoil model in the wind tunnel of the University of Twente reaching a maximum chord-based Reynolds number of 1,000,000 and a free-stream Mach number of 0.15. Measurements are performed to provide a valuable dataset for category 7 of the AIAA workshop on Benchmark problems for Airframe Noise Computations (BANC) as well as for the Hybrid Anechoic Wind Tunnel Workshop (HAWT). Identical hardware and data processing is used aiming at examining purely test section dependent effects. Conventional first-order aerodynamic boundary correction mechanisms are considered along with test-section-dependent acoustic corrections. Acoustically similar aerodynamic conditions are evaluated by considering that the leading-edge slat flow produces identical noise when the slat lift is equal. This is the case when an identical pressure distribution can be observed on the leading-edge slat element. Integrated slat noise levels are similar within ± 2-3 dB for the hard-wall and hybrid test section data. Larger differences are observed when comparing the integrated noise levels with those obtained in the open-jet. In particular we found that the noise scales differently with angle of attack most likely caused by strong jet deflection and streamline curvature resulting in a different flow topology around the leading-edge slat element. Coherence loss effects are found to be relevant at high frequencies in the open-jet.